US4754200A - Systems and methods for ion source control in ion implanters - Google Patents

Systems and methods for ion source control in ion implanters Download PDF

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US4754200A
US4754200A US06/774,110 US77411085A US4754200A US 4754200 A US4754200 A US 4754200A US 77411085 A US77411085 A US 77411085A US 4754200 A US4754200 A US 4754200A
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Prior art keywords
filament
value
arc
cathode
signal
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Frederick Plumb
Christopher Wright
Nicholas J. Bright
Derek Aitken
Bernard Harrison
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Applied Materials Inc
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Applied Materials Inc
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Priority to US06/774,110 priority Critical patent/US4754200A/en
Priority to EP86306924A priority patent/EP0215626B1/en
Priority to AT86306924T priority patent/ATE64492T1/de
Priority to DE8686306924T priority patent/DE3679750D1/de
Priority to JP61212526A priority patent/JPS62143353A/ja
Priority to US06/905,719 priority patent/US4743767A/en
Assigned to APPLIED MATERIALS, INC. reassignment APPLIED MATERIALS, INC. ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: WRIGHT, CHRISTOPHER P., AITKEN, DEREK, BRIGHT, NICHOLAS J., HARRISON, BERNARD, PLUMB, FREDERICK
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/24—Circuit arrangements not adapted to a particular application of the tube and not otherwise provided for
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H01J27/022—Details
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J27/00—Ion beam tubes
    • H01J27/02—Ion sources; Ion guns
    • H01J27/08—Ion sources; Ion guns using arc discharge
    • H01J27/14—Other arc discharge ion sources using an applied magnetic field
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
    • H01J37/08—Ion sources; Ion guns
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/30—Electron-beam or ion-beam tubes for localised treatment of objects
    • H01J37/317—Electron-beam or ion-beam tubes for localised treatment of objects for changing properties of the objects or for applying thin layers thereon, e.g. for ion implantation
    • H01J37/3171—Electron-beam or ion-beam tubes for localised treatment of objects for changing properties of the objects or for applying thin layers thereon, e.g. for ion implantation for ion implantation

Definitions

  • This invention relates generally to ion implantation systems and, specifically, to control of ion source systems in ion implanters. More specifically, this invention relates to systems and methods for controlling a Freeman type ion source in manner which facilitates automation of ion beam generation as well as overall control of an ion implantation system.
  • LSIC large scale integrated circuit
  • Integrated circuit density and speed of operation are both dependent largely upon accuracy and resolution of the lithography and etching systems used to form the patterns of circuit elements in photoresist masking layers on the semiconductor wafer.
  • density and speed are also dependent upon tight control of the profile of doped regions in the semiconductor wafer. Doped regions are regions in which substantial concentrations of conductivity modifying impurities have been added.
  • Tight control of wafer doping can best be achieved using ion implantation techniques and equipment.
  • the uniformity of doping achievable only with ion implantation is critical in the fabrication of smaller geometry devices. It is also critical in the fabrication of devices where a large number of critical implant steps are required. Doping uniformity across individual wafers and repeatability of dopant levels and uniformity from wafer to wafer are critical to high fabrication yields of high density devices. These results can only be achieved using ion implantation.
  • One of the strong desiderata in the field of manufacturing LSIC devices using ion implantation techniques is to improve the wafer throughput capability of the ion implanter while maintaining quality of the implant and without dramatically increasing the cost of performing the implants, especially the cost of performing heavy dose implants which are becoming more popular in the LSIC manufacturing process.
  • the principal parameter which determines wafer throughput in an ion implanter, especially in a heavy implant operation of the system, is ion beam current.
  • the present generation of ion implanters involve a number of different systems of widely varying ion beam current generating capacity, with systems generally being categorized as low current, medium current and high current machines.
  • Freeman ion source to generate a plasma of ions in an arc chamber with an extraction and acceleration electrode system utilized to extract a beam of such ions out of the ion chamber and directed toward an analyzing magnet system to sort out the chemical species in the ion beam of interest before directing the beam onto a target wafer.
  • the Freeman ion source has proven to be a workhorse in the field of ion implantation equipment and is capable of producing arc currents over a wide range of values, it is also well-known that the Freeman ion source is often difficult to control and erratic in its operation.
  • This invention is particularly directed to meeting the performance challenges involved in high current ion implantation systems using Freeman ion sources.
  • High current ion implantation machines in the parlance of the current commercial state of the art are considered to be machines which generate useful boron ion beam currents of about two or three milliamperes (mA) and about ten to twelve mA of arsenic ion beam current. These are the actual ion beam currents striking the semiconductor wafer and it will be appreciated that the arc currents involved in the plasma created within the ion source itself will be at much higher levels and often will involve current levels of one amp or more.
  • the ion source system For flexibility in generating ion beams of different current levels, the ion source system must be capable of stable operation over a wide range of arc currents with different ion species. Moreover, the ion source must be capable of stable operation over the entire typical life cycle of operation of a source with a particular filament.
  • This technology will herein be referred to as the Aitken very high current beam current technology. Ion beam currents at these levels will result in a new generation of ion implantation apparatus which will dramatically improve wafer throughput. Improved technology for wafer scanning for such a new generation of ion implantation equipment is disclosed in a copending and commonly assigned application of Robinson, et al., Ser. No. 774,192 filed 9/9/85 and entitled SYSTEMS AND METHODS FOR ION IMPLANTATION OF SEMICONDUCTOR WAFERS. Advanced techniques for wafer handling in such a new generation of ion implantation equipment is disclosed in a copending and commonly assigned application of Stonestreet, et al., Ser. No. 774,209 filed 9/9/85 entitled SYSTEMS AND METHODS FOR WAFER HANDLING IN SEMICONDUCTOR PROCESS EQUIPMENT.
  • Advanced generations of ion implantation systems will demand increasing degrees of automation of the setup and control of the entire system. Since generation of the ion current in the ion source subsystem of an ion implanter lies at the very heart of the overall system performance, automation of the setup and control of the ion source is a crucial aspect of achieving overall system automation.
  • FIG. 1 illustrates, for purposes of example, the basic ion beam line subsystems of an ion implanter.
  • the particular subsystems disclosed in FIG. 1 show the ion optics geometry of the above-referenced Aitken application.
  • the ion source structure and arrangement itself is a typical Freeman ion source arrangement.
  • FIG. 2 schematically illustrates from a conceptual standpoint, the control of a Freeman ion source utilizing a prior art approach.
  • ions are generated in the arc chamber 15 of the Freeman ion source.
  • An extraction electrode assembly 13 extracts a beam of ions through a rectangular aperture 15A in the front of the arc chamber 15.
  • the ion beam is both extracted and accelerated toward the mass analyzing system 20 which includes an ion beam flight tube 21 providing a path between the poles of analyzing magnet assembly 22.
  • the ion beam is bent in passing through the analyzing magnet assembly 22, enters an ion drift tube 32, passes through a mass resolving slit 33, is accelerated in a post acceeration system 40, and strikes a target element 50.
  • Ion source assembly 11 includes a magnet assembly 12 which has separate electromagnets with cylindrical poles 412A having their axis aligned with the filament 15B within the arc chamber 15.
  • the source magnets produce higher efficiency of ion generation by causing electrons emitted from the filament 15B to spiral around the filament in a path to the anode and thus increase the ionization efficiency in the source.
  • hard driving of the source magnets can affect arc stability.
  • the Freeman ion source is operated from an electrical standpoint by coupling a filament power supply 60 across the filament 15B to supply high current at low voltage to the filament.
  • An arc power supply 61 applies a voltage, which is typically clamped to a maximum of about one hundred and seventy volts, between the filament 15B and the anode 15C within arc chamber 15.
  • Filament 15B generates thermal electrons which are accelerated through the gas species within the arc chamber toward the anode 15C in a spiraling path to create a plasma of the ion species within the arc chamber 15.
  • a separate arc chamber power supply 62 provides a pre-analysis acceleration voltage of up to forty kilovolts to the arc chamber 15.
  • Another separate power supply 63 provides a slightly negative voltage of around minus two kilovolts to extration electrode 13A, also known as the focus electrode.
  • the final electrode 13B is biased at terminal ground so that the final ion beam acceleration is determined by the value of the pre-analysis acceleration voltage applied to the arac chamber itself.
  • the servo control theory implemented in the prior art in an attempt to control and stabilize the operation of the ion source utilizes a constant arc voltage operating mode.
  • An arc voltage error circuit 64 detects the difference between the demanded arc voltage and the actual arc voltage applied between the filament and the anode. Operating in a fast servo loop, the error signal from the arc voltage error circuit 64 controls the arc power supply to maintain the arc voltage at a leverl very close to the arc voltage damand signal.
  • the control of arc current in the prior art approach utilized a feedback loop which controlled filament power.
  • the arc current measuring circuit 65 together with arc current error circuit 66 would attempt to alter the filament power by way of the filament power control circuit 67 to bring the actual arc current back to the level of the demanded arc current.
  • the filament power control loop for maintaining constant arc current is a very slow control loop and is subject to a number of problems and limitations.
  • Another problem with the prior art ion source and extraction system shown in FIG. 2 is the tendency for sparking between the arc chamber 15 and the extraction electrode 13A. It is well known that, because of the dirty atmosphere within an ion source assembly, it is prone to sparking which can have a deleterous effect on the ion beam because the sparking causes the pre-analysis acceleration voltage applied to the arc chamber to drop as the arc chamber power supply runs out of current capacity. This produces a change in the overall beam acceleration voltage so that the preselected chemical species of the ion beam is no longer focused through the resolving slit while the sparking is occurring.
  • One aspect of this invention features a method for operating an ion source having a filament-cathode and an anode mounted within the ion source chamber which includes the steps of supplying direct current electrical power between the anode and the filament-cathode characterized by substantially constant arc current therebetween and varying arc voltage on the filament-cathode and supplying direct current electrical power across the filament-cathode.
  • the value of the arc voltage is monitored and the magnitude of electrical power supplied to the filament-cathode is altered in response to detected changes in the arc voltage to return the arc voltage to substantially a preset reference value.
  • the monitoring step and the altering step are carried out at regular preset intervals.
  • the altering step preferably includes two steps.
  • the first step is to derive a filament power error signal as a prearranged function which includes the difference in values between the monitored arc voltage and the preset reference value multiplied by a predefined integral gain value.
  • the second step is to alter the magnitude of electrical power supplied to said filament-cathode by the value of the filament power error signal.
  • the step of supplying power between the anode and the filament-cathode comprise three separate steps. The first is to monitor the value of arc current flowing between the anode and the filament-cathode. The second is to derive an arc current error signal as a function of the difference between the monitored arc current value and a preset arc current demand signal. The third is to alter the voltage supplied to the filament-cathode as a function of the arc current error signal until the monitored arc current is equal to the arc current demand signal.
  • the method of this invention may be applied to operating an ion source and extracting an ion beam therefrom.
  • An extraction electrode is disposed adjacent the ion source.
  • a deceleration electrode is disposed adjacent the extraction electrode.
  • a direct current electrical potential is supplied to the ion source at a first positive voltage level and a direct current electrical potential is supplied to the extraction electrode at a negative voltage level to extract a beam of ions from the ion source.
  • the method further includes detecting a potential beam latching condition in which the ion beam is striking the extraction electrode to provide a latch detect signal and momentarily altering the voltage supplied to the filament-cathode in response to the latch detect signal to momentarily turn down the arc current to prevent or remedy the latching condition. While the latch detect signal is present, the monitoring and altering steps are interrupted to maintain the filament power value at the value set during the last execution of those steps.
  • Another aspect of this invention features a method for starting and operating an ion source having a filament-cathode and an anode housed in an ion source chamber to reach and maintain a stable preset arc current value and a stable preset arc voltage value.
  • a direct current voltage is applied across the filament-cathode and anode at a initial maximum Varc value in excess of the preset arc voltage value.
  • direct current power is applied to the filament-cathode at an initial value below a power level required to initiate an arc in the ion chamber.
  • Monitoring arc current in the chamber is done at regular preset intervals to determine whether arc current of a preset minimum value is present. At the end of each of the preset monitoring intervals an incrementally increased value of direct current power is applied to the filament-cathode until an arc current of the preset minimum value is detected in the monitoring step.
  • the arc voltage control method discussed above is used to bring arc current to a preset value and then bring arc voltage to a preset value and thereafter to maintain such values.
  • Another aspect of this invention features an ion source operating system which combines an ion source having a chamber with a filament-cathode and an anode mounted within the chamber with an arc power supply arrangement and a filament power supply arrangement of special characteristics.
  • the arc power supply arrangement supplies direct current electrical power between the anode and the filament-cathode and includes a circuit arrangement for maintaining substantially constant arc current flowing between the anode and the filament-cathode with accompanying variations in the arc voltage on the anode.
  • the filament power suply arrangement supplies a value of direct current electrical power across the filament-cathode in response to a filament power control signal.
  • An arrangement is provided for monitoring the value of the arc voltage on the anode; and a further arrangement is provided for developing an filament power error signal as a predefined function of the difference between the arc voltage value and a reference arc voltage value. Finally a circuit arrangement is provided for altering the mangitude of electrical power supplied to the filament-cathode as a function of the filament power error signal to return the arc voltage to the reference arc voltage value.
  • the arc power supply means comprises these three elements: (1) an arc power supply for supplying a dc voltage between said filament-cathode and the anode and responsive to an input control signal to vary the arc voltage on the filament-cathode; (2) a circuit coupled to the arc power supply and the anode for producing an arc current signal corresponding to the value of the arc current; and (3) an error circuit receiving the arc current signal and a preset arc current demand signal for supplying the input control signal to the arc power supply as a signal with a value proportional to the difference between the arc current signal and the arc current demand signal.
  • the arc power supply thereby responds to the control signal to alter the arc voltage until the arc current signal is equal to the arc current demand signal.
  • the altering arrangement comprises five elements.
  • a circuit coupled to the filament-cathode produces a filament current value signal.
  • Another circuit derives a filament voltage signal.
  • a power circuit receives the filament current signal and the filament voltage signal and produces a filament power signal.
  • Another circuit arrangement responds to the filament power error signal and develops a filament power demand signal.
  • an error circuit receives the filament power signal and the filament power demand signal and produces a control signal to the filament power supply with a magnitude proportional to the difference in values between the filament power signal and the filament power demand signal to thereby cause the filament power supply to alter the power applied to the filament until the values of the filament power signal and the filament power demand signal are equal.
  • the overall preferred system embodiment of this invention utilizes programmed computer control of the filament power to achieve arc voltage control with a hardward servo loop maintaining arc current constant in an ion source having a chamber with a filament-cathode and an anode mounted within the chamber.
  • an arc power supply means supplies direct current power between the filament-cathode and the anode and is responsive to an input control signal to control the arc voltage applied to the filament-cathode.
  • An arc current detect circuit coupled to the filament-cathode and the arc power supply produces an arc current signal having a value corresponding to the value of arc current flowing between the filament-cathode and the anode.
  • a first error circuit is provided with an arc current demand input lead and is coupled to the arc current detect circuit means for supplying the input control signal to the arc power supply means as a signal value proportional to the difference between the arc current signal and the signal value on the arc current demand lead to cause the arc power supply to alter the arc voltage until the arc current demand value and the arc current value are equal or the arc voltage is at a maximum value.
  • a filament power supply supplies direct current electrical power across the filament-cathode and is responsive to an input control signal to control the amount of power applied to the filament.
  • a filament power detect circuit is coupled to the filament-cathode for detecting the value of power being consumed in the filament.
  • a second error circuit is provided and includes a filament power demand input lead. This error circuit is coupled to the filament power detect circuit for supplying the input control signal to the filament power supply means as a signal value proportional to the difference between the filament power value and the signal value on the filament power demand input lead sent from the computer to cause the filament power supply to alter the electrical power applied to the filament until the filament power value and the filament power demand value are equal.
  • the programmable computer system includes input circuits communicating with the arc power supply and the filament power detect circuit for inputting values of arc current and filament power, output circuits communicating with the first and second error circuits for outputting values of arc current demand and filament power demand thereto, and memory circuit means for storing values of arc current reference demand and arc voltage reference demand and other predefined variables.
  • the computer system further includes a program arrangement for sending an arc current demand signal value to the first error circuit to establish the arc current at the value and for executing a filament power setting control routine at preset time intervals.
  • This routine includes inputting the value of arc voltage, calculating the absolute value of the fractional error between the input arc voltage value and the stored arc voltage reference demand value, calculating a new filament power demand value as a prearranged function which includes a prearranged gain factor times the difference between input arc voltage and stored arc voltage reference, and communicating the new filament power demand signal to the second error circuit.
  • the features of this invention provide the advantage of very effective control of the ion source and related ion beam extraction components and facilitates total computer controlled automation of this aaspect of ion implanter operation.
  • the invention solves all of the problems which are inherent in prior art control schemes and can be used over a wide range of arc variables of current and voltage.
  • FIG. 1 is a partial section view of an ion implanter beam line which is the preferred system environment for the ion source control system and method of this invention.
  • FIG. 2 is a schematic diagram of a prior art ion source control system and ion beam extraction system.
  • FIG. 3 is a schematic diagram of a system for ion source control in accordance with this invention.
  • FIG. 4 is a block schematic diagram of an ion source control system in accordance with this invention integrated into a total ion implanter computer control system.
  • FIGS. 5 and 6 are electrical circuit schematic diagrams of arc voltage power supply control circuits and filament power supply control circuits incorporating features of this invention.
  • FIG. 7 is a block schematic diagram illustrating the ion source control system of this invention and communication of control and operating parameters to and from a computer system.
  • FIG. 8 is a diagram of the main software routines utilized in an ion source control system in accordance with this invention.
  • FIG. 9 is a software flow diagram for a soft start software routine in accordance with this invention.
  • FIG. 10 is a diagram useful in illustrating the operation of the soft start routine of FIG. 9.
  • FIGS. 11-13 are software flow diagrams of a constant arc current control loop software routine in accordance with this invention.
  • FIG. 14 is a chart useful in explaining a portion of the constant arc current routine shown in FIGS. 11-13.
  • FIG. 3 illustrates in a schematic manner the source control system and method in accordance with this invention which basically involves maintaining substantially constant arc current utilizing a fast servo control loop which alters the arc voltage applied between filament-cathode and anode to produce virtually instantaneous response and control of arc current with a relatively slower servo control loop which detects the changes in arc voltage and controls the amount of filament power to bring the arc voltage back to an arc voltage demand value.
  • This new control approach will be referred to as the constant arc current mode of operation. It is also known as the arc voltage control mode since the filament power control loop is based on arc voltage, not arc current.
  • FIG. 3 also illustrates schematically an anti-latchup system which integrates with the arc power supply control system operating in the constant arc current mode. This portion of the system provides an early detection of an impending beam latchup condition caused by sparking between the arc chamber and the extraction electrode and quickly delatches the beam by pulsing down arc voltage and arc current to quickly shrink the beam and then to quickly recover to normal operating conditions after the delatching function has been accomplished. The details of the operation of this aspect of the invention will be discussed below.
  • the arc voltage control mode of this invention utilizes an arc current detection circuit 70 to detect changes in arc current across a shunt resistor 71 and feeds this arc current signal to an arc current error circuit 72.
  • Arc current error circuit 72 compares the actual arc current with the arc current demand value and outputs a power supply control signal to control the arc voltage output by the arc power supply 61.
  • the output of the arc current error circuit 72 is summed with the output of delatch circuit 90 for overall control purposes. However except when delatch circuit 90 is active, the control of the arc power supply 61 is entirely performed by the arc current error circuit 72.
  • Control of the arc voltage to maintain it substantially constant is provided by a slow control loop implemented in a computer control system 75 which detects changes in arc voltage and executes a control loop program which alters the power demand signal to the filament power supply circuit to cause the power supplied to the filament to change in a direction which will inherently bring the arc voltage back to the demand value.
  • Control of filament power supply 60 is done through a power error circuit 76 which receives the power demand signal from the computer control system 75 and the actual power value from a power compute circuit 77.
  • Power compute circuit 77 computes actual filament power from filament voltage on lead 78 and filament current on lead 79 as calculated by a filament current circuit 80.
  • control algorithm implemented in the computer control system
  • the basic principle of the control scheme involves comparing the actual arc voltage with the arc voltage demand and to use these inputs together with other inputs from the electronic control circuits to generate a new power demand signal which will change the filament power in a direction which will tend to raise or lower arc current so that, interactively, the arc current error circuit 72 will detect this gradual change in arc current and alter the arc voltage accordingly until it is back to the arc voltage demand level.
  • the arc voltage control mode of this invention functioning under the control of computer control system 75, also provides for automated computer controlled startup of the ion source in a soft start routine which brings the source up to a relatively stable standard operating condition before switching over to arc current control mode for finally bringing the operation of the ion source to the arc current and arc voltage demand levels.
  • a soft start routine which brings the source up to a relatively stable standard operating condition before switching over to arc current control mode for finally bringing the operation of the ion source to the arc current and arc voltage demand levels.
  • FIG. 3 also illustrates schematically the general principles of a delatching system in accordance with this invention.
  • the Aitken super high beam current technology utilizes a different beam extraction bias arrangement which incorporates a soft sparking feature.
  • the arc chamber power supply 81 is a hard supply which produces up to twenty kilovolts of pre-analysis acceleration voltage to the ion chamber 15.
  • Extraction electrode power supply 82 is a soft supply which provides up to a negative thirty kilovolts on the extraction electrode 13A.
  • the decel electrode 13B is grounded.
  • any sparking between the arc chamber 15 and the extraction electrode 13A tends to be quickly extinguished because the voltage on the extraction electrode drops rapidly. While this quickly extinguishes the spark and makes it a soft spark in terms of its total energy, the reduction in the voltage on the extraction electrode 13A can also cause the ion beam to lose focus control so that the edges of the beam will begin striking the extraction electrode. When this occurs the beam can latch up in this condition due to the inability to recover the focus voltage even when the spark has disappeared.
  • the delatching circuit of this invention employs a current level detect circuit 83 which detects the amount of current being drawn by the extraction electrode power supply and produces a corresponding output voltage. This output voltage is compared with a reference voltage in a latchup dietect circuit 84 which produces an output when the output of the current level detect circuit 83 exceeds the reference voltage value, indicating a latchup condition.
  • This latchup signal is set by way of the fiber optic transmitter 85 to a fiber optic receiver 86 to signal a delatch circuit 90 to send an output to the arc power supply which quickly pulses down the arc voltage to momentarily turn down the ion beam and thereby eliminate the beam latched condition.
  • the delatch circuit 90 sends a status signal to computer control system 75 to advise of the latched condition so that changes in arch current and voltage during delatching may be ignored.
  • FIG. 4 together with FIG. 1 illustrates the manner in which the ion source control system and method of this invention may be integrated into an overall computer control arrangement for all of the subsystems of an ion implanter.
  • the arc voltage control mode of this invention can be utilized in any ion implantation system which utilizes an ion source based on a heated filament and a filament-cathode to anode acceleratio potential which basically has the same type of operating characteristics as the Freeman ion source.
  • the ion source control system and method of this invention can be employed in ion implanters of the prior art as well as in an ion implanter based on the Aitken super high current beam technology.
  • the preferred environment for use of the ion source control system and method of this invention is an advanced ion implantation system which utilizes beam line technology from the above-identified copending Aitken application and further utilizes, as the wafer target scan system 51 and the wafer loading system 53, the technology disclosed in the above-identified copending patent applications.
  • These technologies together with the system and method for ion source control in accordance with this invention facilitate bringing all operational aspects of an ion implanter under control of a central computer control system 75.
  • the pre-analysis acceleration voltage applied to the arc chamber by the extraction system power supplies controls the ion beam velocity extracted from the arc chamber 15.
  • the ion beam analyzing system 20 is a mass or momentum analyzer, the current supply to the electromagnets of the beam analyzing system must be controlled in terms of the ion energy entering the beam analyzing system so that ions of the desired chemical species are passed through the resolving system 30 into the post acceleration system.
  • Control of the overall ion source involves control of the ion source gas feed system 16 (which may include a direct gas feed and ovens for vaporizing solid sources) and the ion source magnet system 12 as well as control of the electrical operating parameters of the filament-cathode and anode within the arc chamber 15. Since control over generation of the ions in the ion source lies at the very heart of the implanter system, achievement of effective computer control of the ion source itself is the foundation on which full automation of the entire implanter can be built. It has further been discovered that the ion source control system and method of this invention permits advanced implanter features such as automatic source tune-up under computer control to be realized. Using this inventio, small beam currents can be controlled using small arcs rather than generating large currents and using vanes to reduce beam current to small values.
  • FIGS. 5 and 6 are partial circuit schematics which illustrate in more detail the circuit implementation of the electronic hardware portion of this invention.
  • the filament power control signal sent by the computer control system 75 to the filament power error circuit 76 is transmitted as a multibit digital control signal to the data acquisition unit associated with teh filament power control system and a digital to analog converter 76A converts the digital signal to an analog signal for input to the filament power error circuit.
  • various analog signal values of filament current, filament voltage, filament power, filament power control and digital to analog converter output are fed back to the data acquisition unit, typically through appropriate signal coupling networks, so that the central computer control system 75 can monitor the control signal parameters and operating parameters for purposes of status checking and error analysis.
  • FIG. 6 illustrates in more detail the arc power supply and control circuits associated therewith.
  • the version shown in FIG. 6 includes a mode realy for controlling whether the arc voltage or arc current control mode is executed. It has been determined that the arc voltage control mode (i.e., arc voltage control of filament power with constant arc current) is superior over all ranges of arc conditions and thus the mode switch need not be provided.
  • the arc voltage control mode i.e., arc voltage control of filament power with constant arc current
  • the arc demand analog signal is input on a terminal 100 as the output of a motor potentiometer circuit which is set under computer control. Since this arc demand signal is typically set once and does not require fast alteration, motor potentiometer control is satisfactory. This contrasts with the rapid digital control signal communication of a new power control signal from the computer into the filament power supply control circuit shown in FIG. 5. However, under certain circumstances it may be desireable to also be able to rapidly change the arc demand signal. Consequently, a separate provisio is made for communicating an arc demand digital signal component via digital input lines 101 to a digital to analog converter 102 whose output is summed with the motor pot analog demand signal at the input to the overall error circuit 72, 73.
  • the analog input signal summing arrnagemnet shown in FIG. 6 simplifies the overall error circuitry in view of the number of different input analog signals to be utilized in the overall final arc voltage control signal.
  • the delatch circuit 90 receives a latch in signal and couples that signal at an appropriate analog level to the summing input of the error amplifier circuit to temporarily override any other input control voltages to perform the beam delaatching function.
  • a hardware overcurrent protect circuit 103 is also included in the hardware control system. This overcurrent circuit provides a hardware clamp on arc current so that dangerously high arc currents cannot be called for.
  • FIG. 7 illustrates in somewhat greater detail the input and output signals which are communicated via data acquisition units to the filament control system and the arc power supply control system.
  • the ion source control system of this invention is preferably implemented in an ion source of the type disclosed in the above-referenced Aitken patent application which utilizes a split anode arrangement with each of the separate anode sectios 110A, 110B and 110C being coupled to separately programmable power supplies 111A, 111B and 111C for purposes of controllably modifying the plasma distribution in the ion source chamber and to compensate for field modification effects produced by the voltage applied across the filament.
  • Motor pot 112 is controlled via the DAQ system 113 which may involve one or data acquisition modules and separate motor pots 113A, 113B and 113C are operated to control the individual anode section power supplies 111A, 111B and 111C.
  • a separate data acquisition module 114 is used to communicate with the filament power supply control board and has basically the input and output signals indicated.
  • FIG. 8 illustrates the basic software routines involved in a constant arc current mode of controlling the Freeman type ion source in accordance with this invention. Since the constant arc current routine can be entered when the arc in the arc chamber is already on, the first test performed in to determine whether the arc is on or not. If the arc is on, the system directly enters the constant arc current control loop routine. If the arc is not on, the first routine performed is one to switch on the arc voltage power supply and the filament power supply. After this switch-on routine has been performed a soft start routine is performed to initiate an arc in the arc chamber in a well-controlled manner. If the soft start routine is successful, the constant arc current control loop is entered.
  • the routine to switch on the power supplies is relatively simple and need not be discussed in any detail here. Basically the routine starts by making sure the Freon coolant is turned on to the arc chamber and the dummy load resistor relay at the output of the arc power supply is off. A one amp current demand is set into a data structure and then the arc current motor pot is adjusted to this stored demand value. Thereafter, the mains power is switched on to the arc power supply, a timer is set and a control loop entered to check whether the mains has actually turned on prior to timing out of the timer.
  • a similar routine is used to turn on the filament power supply.
  • Filament power demand is set at a minimum value, for example 120 watts, and then the power mains input to the filament power supply is turned on.
  • a similar timer and checking loop is used to determine whether the mains to the filament power supply turns on within a predetermined time. Thereafter the system checks to determine that the filament supply is actually on.
  • the software enters the slow start routine shown in FIG. 9.
  • the basic function of the slow start routine is to slowly ramp up the filament power in small increments until an arc is struck and minimum arc current is flowing. This result must be achieved before a timer times out and before filament power is ramped up to the maximum permitted filament power.
  • the actual steps in teh slow start routine will now be described with reference to FIG. 9.
  • the first step is to set a timer to 100 seconds as the maximum time permitted for successful striking of an arc.
  • the routine pauses for 0.2 seconds and then gets the values of the arc current and the filament power via the DAQ communication system as previously discussed.
  • a test is performed to determine whether the filament power is greater than or equal to the m;aximum permitted filament power. If it is, the timeout value is forced TRUE so that the routine will later exit and not loop around.
  • the next step is to set the arc filament power (ARC.FIL.PWR) value, i.e. the value to be demanded for arc filament power to be equal to the actual filament power plus an incremental 25 wats.
  • the 25 watt increment is a convenient value which has proven to be practicable but other incremental values in about the same range could be utilized.
  • the system next determines whether this new arc filament power demand value is less than the arc power minimum value (e.g., 120 watts) already set into the filament control system during the supplies turn on routine. If it is, the arc power minimum value is maintained unchanged. Referring to FIG. 10, it can be seen that the arc filament power signal remains at the 120 wall value initially set unitl a new value overrides it.
  • the ARC.FIL.PWR demand signal is sent via the DAQ to the filament power control circuit as the new filament power control signal.
  • the system checks to see whether a timeout has occurred. If the timeout has occurred, the system then goes off to a branch which determines what type of error occurred in striking the arc. This will be discussed below.
  • the system tests to determine whether the value of arc current is greater than the minimum arc current value which has been established by experience to be a value which signals that an arc has been successfully struck.
  • This value may, for example, be 100 mA as shown in The diagram of FIG. 10. If this test returns a negative, as it will be during the first few loops through the slow start routine, execution of the routine branches back to the pause step and re-executes. Under normal circumstances, after some number of loops through the main circumstances, after some number of loops through the main routine, an arc will be struck and the arc current will rise above the minimum arc current value and software control will be returned to enter the constant arc current control loop.
  • the routine executes the step of reading the arc voltage and then testing to see whether it is less than maximum arc voltage of 140 volts. If the arc voltage is less than the maximum 140 volts, the error declared is in the arc striking voltage itself. If the arc voltage is at the maximum of 140 volts, the error delcared is that the arc simply failed to strike. With one or the other of these machine errors delared, the user will be signaled that some problem has occurred and an attempt to locate the problem can thereafter be made.
  • the constant arc current control loop takes over to bring both the arc voltage and arc current to the user-defined or machine-defined values and thereafter to maintain the arc voltage at a substantially constant value utilizing a slow software-based servo loop which basically looks to detect a change in the arc voltage and to correspondingly change the arc filamaent power to bring the arc voltage back to the demand value.
  • the constant arc current control loop routine is illustrated in FIGS. 11-13 which will be described in conjunction with the graph shown in FIG. 14.
  • the constant arc current routine begins by initializing certain variables utilized in the routine.
  • the STABLE COUNT variable is set to zero as are the NO.ARC and Vref.OLD variables.
  • the valaue of the Vold variable is set to the current arc voltage value:Varc read fromthe circuitry.
  • the routine After the variables haavea been initialized, the routine first tests to see whether the ion beam is in a latched condition. It does this by looking at a data structure flag which is written each time the beam latches. If the beam is in a latched condition, the routine branches off for three-tenths of a second and then checks again to see if the beam is still latched.
  • the system reads in the demanded arc current Iref and the demanded arc voltage Vref. These values will be stored in internal data structures within the computer. The system then gets the value of the arc voltage Varc via the appropriate DAQ.
  • the next step performed is to determine whether the demand arc voltage Vref has been set to a zero value, which is not realistic. If such a value has been set the Vref value is reset to a default value Vdef and execution continues.
  • the next step is to test whether the value of Vref is equal to the value of Vref.OLD. It will be remembered that Vref.OLD was initialized to zero as the first step in the program, so the first time through the loop, this test will be negative. Thiscauses the program to set MIN.FIL.PWR equal to 200 watts, MAX.FIL.PWR equal to 1000 watts, and to set Vref.OLD equal to Vref. In subsequent loops through the routine Vref will equal Vref.OLD unless a new Vref value has been set into the data structure either by the user or some automatic source tuning routine.
  • the system now goes out and gets the values of FIL.PWR and arc current Iarc.
  • the Iarc value is tested to see if it's less than 25 mA.
  • This 25 mA value is one that has been preselected as a practicable value to use as a test whether the arc may be out under some circumstances under which not a final test since there are circumstances under which the arc current demand may be set at less than 25 mA. However, in most operating situations higher arc current will be demanded and the arc will probably be out if the arc current value is below 25 mA. If this test is positive the system branches off to special routines to deal with that situation. These routines will be described below.
  • the first step in this routine is to calculate a fractional error ER as the absolute value of the difference between Varc and Vref divided by Vref.
  • the integral gain constant is set equal to the fractional error ER times a preset gain constant value of 0.5 which has been empirically determined ot give good results.
  • the system determines whether the fractional error is so small that it can be ignored for purposes of a differential gain factor in the final change in ARC.FIL.PWR.
  • the system tests whether than ER is less than or equal to 0.1. If it is, the differential variable DV is set to zero. Otherwise, the differential variable DV is set to a value equal to Varc minus Vold.
  • the system then goes into a checking routine to see whether it makes sense to use the new ARC.FIL.PWR value calcualted. It first tests whether the new ARC.FIL.PWR value is less than the MIN.FIL.PWR value. If that test is affirmative, ARC.FIL.PWR is set to the minimum value rather than use the calculated value. This precludes setting a new ARC.FIL.PWR value which could cause the arc to go out. Next a test is performed to determine whether the new ARC.FIL.PWR value is greater than the MAX.FIL.PWR value. If this test is negative, the new ARC.FIL.PWR value is sent via the DAQ to the filament power control circuit to move the filament power in a direction which will tend to correct the error in arc voltage.
  • the program sets the ARC.FIL.PWR value equal to the MAX.FIL.PWR value and then tests to see whether the Varc value is greater than the minimum arc striking voltage of 140. If this test is satisfied, the arc is in a highly unstable condition. Therefore, to try to recover a stable condition, or at least to prevent burnout of this filament, the arc filament power is set at a maximum of 500 watts. If the above-stated condition is not satisfied, the system calcuates the maximum power the filament is capable of taking MAX.FIL.P using the resistance value Ifil/Vfil multiplies by the square of the value of the maximum voltage deliverable by the filament power supply.
  • the system then tests to see of the value of MAX.FIL.P is greater than the ARC.FIL.PWR (which was set to MAX.FIL.PWR). If it is greater, then the ARC.FIL.PWR is set to the MAX.FIL.P value. If it is not greater, the ARC.FIL.PWR value remains at MAX.FIL.PWR. At this time the ARC.FIL.PWR value is sent to the filament power control circuit as the new power demand signal.
  • the program performs a check to determine whether the arc is and has been in a very stable condition and, if it has, it narrows the MAX.FIL.PWR-MIN.FIL.PWR band to plus and minus 30 watts offset from the current ARC.FIL.PWR value as shown in FIG. 14. Specifically the program first tests whether the ARC.FIL.PWR value is equal to the ARC.FIL.PWR. OLD value. If this test return a NO, the program sets the STABLE COUNT variable to zero, sets the MIN.FIL.PWR to 200 watts and the MAX.FIL.PWR to 1000 watts.
  • the system tests whether the STABLE COUNT value is greater than or equal to the stable threshold parameter which has been selected to be 25. If the STABLE COUNT is greater than or equal to 25, MIN.FIL.PWR is set equal to ARC.FIL.PWR -30 and MAX.FIL.PWR is set to ARC.FIL.PWR +30. These values will prclude large changes in the next ARC.FIL.PWR calculatio during the next time through the loop. This is based on experience that, if the arc has been stable for some time, a condition which makes it look unstable may be a temporary phenomenon such as a small flake falling into the source and temporarily shorting out the arc or a beam latching condition.
  • the program pauses for 0.2 seconds to allow whatever change in arc filaament power demand has been made to take effect. It then sets the ARC.FIL.PWR.OLD value to the current ARC.FIL.PWR value to use the next time around the loop and then the program loops back to the beginning to re-execute.
  • the system first tests to see whether the NO.ARC variable is greater than or equal to 3. In other words, the system is given three times through the loop to recover itself from what appears to be a no arc condition. If that test returns a negative, the next step is to test whether Iref is greater than Iarc. If Iref is not greater than Iarc, this means that Iarc has been set at a value less than 25 mA and the arc is not out. Consequently the value of NO.ARC is set to zero and the programs continues its execution at point D.
  • the system tests to see if the Varc value is greater than or equal to the minimum arc striking voltage of 140. If that test returns a NO, it means the arc ahs not gone out and NO.ARC is set to zero. If that test returns a YES, the NO.ARC value is incremented by 1.
  • the ion source control system and method of this invention provides an effective approach to controlling the arc conditions within a Freeman type ion source (or other similar type ion source). It will be apparent to those skilled in the electronic circuit and software art that various changes could be made in the systemand method as disclosed without departing from the basic concepts on which this invention rests. For exmaple, it would be possible to have the computer contorl system calculate actual filament power rather than computing it in an analog fashion in the circuitry. If has been determined that the overall efficiency and effectiveness of the system is enhanced using an analog compute of the actual filament power. Furthermore this reduces the amount of communication that is required to the filament power supply control board.
  • the system and method of this invention has been verified to provide very good results in actual testing in prototype ion implanter systems utilizing the Aitken super high beam current technology. Stability of ion source operation has been achieved over arc current values ranging from 10 mA to 10 amps.
  • the high arc current region of operation is especially important for the Aitken super high beam current technology since the ion source is being driven very hard under these conditions and is most subject to erratic behavior and difficult control operations under there high arc current conditions.
  • these conditions require that high source magnetic fields be utilized for efficiency in creating the plasma within the arc chamber. This also tends to create instability in the operation of the ion source.
  • the ability to control low arc currents is also important since it facilitates running low beam currents without beam vane trim-out of a large fraction of the beam current.
  • the overall ion implanter system functioning under computer control can utilize various other software algorithms for implementing such advanced features as automated source tuning for maximum beam current or to achieve other targeted beam parameters.
  • the main advantage of the system and method of this invention is that it will facilitate overall automated operation of an ion implanter to a degree that the clean room operators of the implanter will be able to select ion implant procedures from a menu of prearranged stored ion implant protocols, load the wafers into the system and set the system to running on automatic operation without further tending by technicians or operators. Accordingly, this invention contributes directly and materially to the achievement of fully automated io implanters which are needed in advanced semiconductor integrated circuit fabrication operations. It will be appreciated by those skilled in the art that numerous changes could be made in the specific system and method embodiments of this invention disclosed herein without departing from the scope of the invention as claimed in the following claims.

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US06/774,110 1985-09-09 1985-09-09 Systems and methods for ion source control in ion implanters Expired - Lifetime US4754200A (en)

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US06/774,110 US4754200A (en) 1985-09-09 1985-09-09 Systems and methods for ion source control in ion implanters
EP86306924A EP0215626B1 (en) 1985-09-09 1986-09-08 Systems and methods for ion source control in ion implanters
AT86306924T ATE64492T1 (de) 1985-09-09 1986-09-08 Vorrichtungen und verfahren zur steuerung der ionenquelle in ionen-implantierungsgeraeten.
DE8686306924T DE3679750D1 (de) 1985-09-09 1986-09-08 Vorrichtungen und verfahren zur steuerung der ionenquelle in ionen-implantierungsgeraeten.
JP61212526A JPS62143353A (ja) 1985-09-09 1986-09-09 イオンインプランテ−シヨン装置のイオンソ−スを制御する装置及び方法
US06/905,719 US4743767A (en) 1985-09-09 1986-09-09 Systems and methods for ion implantation

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EP0777256A2 (en) 1995-11-23 1997-06-04 Applied Materials, Inc. Deceleration after mass selection for ion implantation.
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US6703627B2 (en) * 2002-07-24 2004-03-09 Taiwan Semiconductor Manufacturing Co., Ltd Real time monitor method and system for extraction electrode
US20040016893A1 (en) * 2002-07-24 2004-01-29 Taiwan Semiconductor Manufacturing Co., Ltd. Real time monitor method and system for extraction electrode
US20050173651A1 (en) * 2003-10-24 2005-08-11 Applied Materials, Inc. Cathode and counter-cathode arrangement in an ion source
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JPH0563894B2 (ja) 1993-09-13
JPS62143353A (ja) 1987-06-26
EP0215626A3 (en) 1988-07-27
EP0215626B1 (en) 1991-06-12
EP0215626A2 (en) 1987-03-25
DE3679750D1 (de) 1991-07-18
ATE64492T1 (de) 1991-06-15

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